1. Technical Field
The present invention relates to an exposure head that exposes a surface of an object to light emitted by a light emitting segment, an image forming apparatus using the exposure head, and an image forming method using the exposure head.
2. Related Art
JP-A-2004-195963 describes an exposure head that exposes a surface such as a surface of a photosensitive drum to form a latent image on the surface. The exposure head has multiple light emitting segments. Light emitted by the light emitting segments is incident on the surface and forms spots on the surface. As a result, an image is formed on the surface. The surface is uniformly charged to a certain potential before the exposure by the exposure head. Portions of the surface, on which the spots are formed, are discharged by the exposure so that a desirable latent image is formed on the surface. Then, charged toner is deposited on the discharged portions so that the latent image is developed into a visible image.
As described in JP-A-2004-195963, organic electroluminescence elements may be used as the light emitting segments. This type of light emitting segment generates heat when the light emitting segment emits light. In addition, the intensity of light emitted by this type of light emitting segment may vary due to a variation in the temperature of the light emitting segment. Thus, this type of light emitting segment has the following problem.
The light emission state of each light emitting segment included in the exposure head depends on a latent image to be formed. Specifically, when a latent image is to be formed for a high-density image, the frequency of light emission by each light emitting segment is high. On the other hand, when a latent image is to be formed for a low-density image, the frequency of light emission by each light emitting segment is not high. It is assumed that a latent image to be formed includes both a portion for a high-density image and a portion for a low-density image. In this assumption, some of the light emitting segments frequently emit light to the portion for the high-density image and thereby have high temperatures. However, the other light emitting segments do not frequently emit light to the portion for the low-density image and thereby have relatively low temperatures. Thus, the light emission state of each light emitting segment depends on the latent image to be formed. As a result, the temperatures of the light emitting segments may vary. Due to a variation in the temperature of each light emitting segment, the intensity of light emitted by the light emitting segment varies. A difference between or differences among the temperatures of the light emitting segments leads to a difference between or differences among the intensities of light emitted by the light emitting segments. Therefore, a failure may occur in a formed image. Specifically, an unwanted difference between or differences among gray levels may occur in the formed image.